My brother has worse immigration history & he was overstay , and yesterday his visa approved. He applied as Partner of British Citizen.
He submit docs of last 2 years living together , marriage certificate, children birth certificates, letter from school, letter from GP etc
No question were asked, it was simple and straightway.
You can take any complex case for Same Day Services as far as its FLR(FP) route and you have all your documents in hand.
I would recommend people who are waiting for long time for their decision pending or they are planing for a postal application, please use Same Day Services .... its worth spending £600 extra. however you need your Original Passport. My brother previous passport was held by home office, so he applied for new one.
Any questions.. please let me know.